Probing the Structure of Organosilane Films by Solvent Swelling and Neutron and X-ray Reflection

Abstract

Thin films of organosilanes have great technological importance for adhesion promotion, durability, and corrosion resistance. Although the cross-link density profile within such films is likely to strongly affect their performance, no reliable assay has been available to characterize this distribution. In this work we use solvent swelling combined with neutron and X-ray reflection to study the cross-link density distribution within ultrathin films (50−100 Å) of (3-glycidoxypropyl)trimethoxysilane (GPS) and bis(triethoxysilyl)ethane (BTSE) on silicon. The films are swelled with vapors of the good solvent nitrobenzene (NB). For GPS films, the solvent concentration profile has a strong maximum in the central region of the film. We conclude from this that the GPS films possess a high cross-link density near the silicon surface and a much lower cross-link density in the bulk of the film. The decreased solvent concentration at the air surface is more difficult to interpret. It appears to also indicate an elevated cross-link density, but may also involve contributions from other effects such as the difference in surface tension between GPS and NB. The degree of swelling reaches ∼50−60% in the bulk of the films. Good reproducibility is obtained for several swelling/deswelling cycles, indicating that swelling with d-NB does not irreversibly change the structure of the network. In contrast to GPS, highly cross-linked films of BTSE do not swell when exposed to d-NB, even though NB is also a good solvent for the BTSE monomers.